An electro-photographic printer performs printing by forming a latent image on a photosensitive drum through exposure of the drum, developing this latent image with toner, transferring the toner onto a sheet of paper and fixing the toner on the paper by means of heat or the like.
An exposure process for forming a latent image is classified into an LED optical system and a laser optical system. In a write head (hereinafter, referred to as an optical write head) of an LED optical system, light outputted from the LED is irradiated onto a photosensitive drum through an erecting unit magnification rod lens array.
FIG. 1 is a sectional view taken perpendicularly to the longitudinal direction of an optical write head to be mounted on a conventional electro-photographic printer. In this optical write head, a plurality of light-emitting element array chips 64 each having light-emitting element arranged in line are mounted in the scanning direction on a substrate 63 and an erecting unit magnification rod lens array 61 which is longer in the scanning direction is fixed at a position on an optical path of light outputted by light-emitting elements of the light-emitting element array 64 by a housing 62 made of resin. And the outer edge portions perpendicular to the longitudinal direction of the substrate 63 are engaged with the leg end portions of the housing 62. In addition, a heat sink 60 for radiating heat of the light-emitting element array chips 64 is provided under the substrate 63, and the housing 62 and the heat sink 60 are fixed by metal retainers 66 with the substrate 63 between them.
A photosensitive drum 65 is provided above the rod lens array 61. The rod lens array 61 forms a latent image on the photosensitive drum 65 by collecting light of light-emitting elements of the light-emitting element array chips 64 and exposing the photosensitive drum.
An erecting unit magnification imaging optical system can be very compact by using an optical write head as described above. Since a rod lens array is manufactured by arranging and fixing a number of rod lens elements with resin, however, some irregularity or the like in arrangement of rod lens elements is liable to occur. Such irregularity in arrangement has an influence on the resolution of a rod lens array and makes a cause of image irregularity and the like in a recent machine of high resolution (resolution of 1,200 dpi for example).
In order to suppress the occurrence of such an arrangement irregularity, it is conceived to substitute a resin lens array for a rod lens array as described above. A resin lens array, which realizes an erecting unit magnification imaging optical system by stacking one over another two or more flat lens array plates each having a number of single lenses formed on a transparent substrate, has no possibility of making any arrangement irregularity in the lens array thanks to manufacturing a lens array plate having a number of single lens formed in it by injecting resin into a mold.
An optical write head used in an electro-photographic printer mounted with such a resin lens array is disclosed in Japanese Patent Laid-Open Publication No.2000-221, 445. The optical write head disclosed in this official gazette is a head of an erecting unit magnification optical system improving the resolution by stacking one over another a plurality of resin-molded lens array plates.
However, an optical write head disclosed in Japanese Patent Laid-Open Publication No.2000-221, 445 has the following problems.
In case of stacking a plurality of lens array plates one over another, it is necessary to align them so that the opposing vertexes of lenses are made small in deviation of position relative to one another thereof. In order to use a resin lens array in a high-resolution optical write head, it is necessary to make the size of a single lens as small as possible, and for example in a resin lens array to be used in an optical write head having a resolution of 1200 dpi, a single lens needs to be 0.4 mm or less in diameter according to simulation and naturally the alignment of lenses must be adjusted accurately in the order of micrometers. Therefore, a high accuracy is required for the alignment of lenses and in a structure where a plurality of lens array plates are stacked on one another, the alignment of them is very difficult.
And since a developing roller which discharges toner is located near the output surface of a resin lens array, foreign matters such as toner and the like are floating there and the foreign matters stuck to the output surface of the resin lens array cause deterioration in performance of the optical write head, and therefore removal of the stuck foreign matters is periodically performed by a method of wiping off with soft cloth or the like. However, since a resin lens array using convex lenses has a surface which is depressed and protruded in shape, it is difficult to clean. And since resin generally used in an optical lens is so comparatively soft as to be B to HB (JIS K5401) in pencil hardness, the surface of a resin lens array is liable to be scratched when stuck matters are removed and there is the possibility of deteriorating the optical performance of it.
And resin used generally in an optical lens is 90 to 93% in transmittance (ASTM D1003) and since a lens array which is as bright as possible is needed for high-speed printing, a resin lens array is desired to be made as thin as possible but thinning it leads to remarkable degradation of a shape-retaining ability of the resin lens array itself. Mounting a resin lens array having a poor shape-retaining ability on a head with a high positioning accuracy leads to more complication and higher precision in structure of members supporting the resin lens array and results in increasing the cost of components.
And since resin used in an optical lens is generally high in coefficient of thermal expansion, in case of sticking and fixing a resin lens array to a housing formed out of a rigid material, the occurrence of strain caused by the difference in coefficient of thermal expansion between the resin lens array and the housing deforms the resin lens array and thereby lowers the accuracy of position of the resin lens array. And since in a resin lens array an adhesive agent is generally used for fixing lens array plates to each other, the slippage in stacking position caused by exfoliation in the adhesive interface between the lens array plates deteriorates remarkably the quality of image.
And since a resin lens array is low in rigidity and has a poor self-shape-retaining ability, to fix the resin lens array highly flat, it needs to be fixed in position along a datum plane of another member which is high in flatness. In this case, for example a method of attaching a resin lens array to a datum plane of a housing and fixing the housing and the resin lens array to each other with an adhesive agent is conceivable, but this method needs to keep the whole resin lens array attached to the housing pressed into the housing side until the adhesive agent is hardened so as to be fixed along the datum plane and requires a considerably long time in the manufacturing process.
And when a light beam reaches the interface between transparent media which is different in refractive index from each other, a part of the light beam is reflected by the interface and the other parts pass through the interface and enters the next medium. The quantity of reflected light at an incident angle of 0° between an optical resin lens and an air layer is about 4%. That is to say, the transmittance in case that a light beam passes through a single lens array plate is:(1−0.04×2)×100=0.92×100=92 (%).In an optical system in which n lens array plates are stacked one over another, since a light beam passes through the n lens array plates, the total transmittance becomes (0.92n×100) %. This means using a plurality of lens array plates reduces the total quantity of transmitted light. For example, the total transmittance of a structure in which lens array plates of three layers and a protective cover of one layer are used is 0.924×100=72% and results in bringing about the reduction in quantity of light of 28%.